1. Art Field
The present invention relates to a multilayer thin film including a ferroelectric thin film and an electron device comprising such a multilayer thin film. Typically, the multilayer thin film may be applied to semiconductor memories, thin-film ferroelectric devices such as infrared sensors, recording media for recording information by polarization reversal of ferroelectrics by AFM (atomic force microscope) probes or the like, thin-film vibrators, thin-film VCOs and thin-film filters used for mobile communications equipment, thin-film piezoelectric devices used for liquid injectors or the like.
2. Background Art
Electronic devices comprising dielectric films, ferroelectric films, piezoelectric films or the like formed and packed on Si substrates or semiconductor crystal substrates have been invented and intensively studied. For instance, LSIs having ever higher packing densities and dielectric separated LSIs using SOI technologies have been invented through combinations of semiconductors and dielectric materials, semiconductors storage devices such as nonvolatile memories through combinations of semiconductors and ferroelectric materials, film bulk acoustic resonators or FBARs, thin-film VCOs, thin-film filters, etc. through combinations of semiconductor substrates and piezoelectric films.
To allow such electron devices to have the optimum device performance and its reproducibility, it is desired that single crystals be used as dielectric materials, ferroelectric materials and piezoelectric materials. The same goes for thin-film materials. With polycrystal materials, it is difficult to obtain satisfactory device performance for the reason of physical quantity disturbances due to grain boundaries, and so epitaxial films as close to perfect single crystals as possible are now desired. A requirement for FBAR devices is that they be formed on Si single crystal substrates because the substrates should be processed with high accuracy. In addition, when ferroelectric materials such as PZT are used as FBAR materials, it is believed that the largest output is obtained when the spontaneous polarization of a ferroelectric material lines up in one direction. For this reason, it is ideally desired that a (001) uniaxially oriented ferroelectric thin film be formed on an Si single crystal substrate by epitaxial growth.
Typical ferroelectric thin films include those of PbTiO3, PZT, BaTiO3, etc. To apply these perovskite oxide thin films to actual devices, it is required to form them on semiconductor substrates. However, it is very difficult to form a uniaxially oriented ferroelectric thin film such as a (001) uniaxially oriented BaTiO3 film of good crystallographic properties on a semiconductor substrate such as an Si (100) substrate. To overcome such difficulty, the inventors have filed patent applications (JP-A 09-110592, etc.) to come up with a process wherein a ferroelectric epitaxial thin film can be easily formed on an Si single crystal substrate.
Usually, however, a ferroelectric thin film formed on an Si substrate as an example have properties vastly inferior to those derived from ferroelectric""s own properties. The properties of a ferroelectric material, e.g., dielectric constant, Curie temperature, coercive electric field and residual polarization change with stresses that the ferroelectric material has. A thin-film form of ferroelectric material is likely to generate stresses in association with film formation, and so stress control is of importance to form a ferroelectric thin film having improved properties. Stresses in particular have a great influence on the deterioration of the properties of a ferroelectric thin film formed on an Si substrate.
For instance, J.A.P. 76(12), 15, 7833 (1994) and A.P.L. 59(20), 11, 2524 (1991) teach that two-dimensional stresses in a film plane have a strong influence on the properties of a ferroelectric material on an MgO single crystal substrate, not an Si single crystal substrate. A leading cause for stress generation is a difference between the underlying substrate and the ferroelectric in physical properties, e.g., the coefficient of thermal expansion and lattice constant. To apply a ferroelectric thin film to a device, therefore, any desired ferroelectric properties cannot be stably obtained without such stress reductions as mentioned above.
Here ferroelectric materials having preferred properties include Pb-base ferroelectric materials such as PbTiO3, PLT (PbTiO3 with La added thereto), PZT (PbZrO3xe2x80x94PbTiO3 solid solution) and PLZT (PbZrO3xe2x80x94PbTiO3 solid solution with La added thereto). The Pb-base ferroelectric materials, for the most part, have their axes of polarization in the [001] direction; they should preferably be uniaxially oriented films in terms of ferroelectric properties. When a Pb-base ferroelectric thin film is formed on an Si single crystal substrate, however, a domain structure is likely to occur, in which structure (001) oriented crystals coexist with (100) oriented crystals.
Set out below is one possible reason why the Pb-base ferroelectric domain structure is easily formed on the Si single crystal substrate. In what follows, PZT is used as an example of the Pb-base ferroelectric material.
Si is much smaller in the coefficient of thermal expansion than PZT. Accordingly, if a PZT thin film is formed at a temperature of 600xc2x0 C. for instance, the contraction of the PZT thin film is then disturbed by the Si substrate in the process of cooling the thus formed thin film down to room temperature and, as a result, relatively large two-dimensional tensile stresses are generated within the plane of the PZT thin film. To make up for such tensile stresses, PZT must be forcibly formed into a 90xc2x0 degree domain structure film in which (001) oriented crystals coexist with (100) oriented crystals. As the PZT thin film is cooled down, the tensile stresses remain generated therein even after domain formation and so the ferroelectric properties thereof become low.
The same holds true for the case where the PZT thin film is used as a piezoelectric material. To enhance the piezoelectric properties of the PZT thin film, it is of importance to increase the proportion of the (001) oriented crystals as much as possible, and to reduce the tensile stresses on the PZT thin film as much as possible.
On the other hand, the inventors have proposed a process for obtaining a ferroelectric thin film having a tetragonal (001) orientation while making use of elastic distortion resulting from a difference in lattice constant between both, called a misfit, as set forth in JP-A""s 10-223476 and 11-26296, wherein a perovskite oxide thin film is formed on an electrically conductive oxide thin film. With this process, it is possible to form on an Si (100) substrate a (001) uniaxially oriented ferroelectric thin film of several tens of nanometers in thickness.
IEEE ELECTRON DEVICE LETTERS, Vol. 18 (1997), pp. 529-531, Jpn. J. Appl. Phys. Vol. 137 (1988), pp. 5108-5111 and JP-A 11-274419, too, describe that as in the aforesaid process, a perovskite oxide such as BSTO is formed on an electrically conductive oxide such as SrRuO3, whereby a dielectric film is elongated in the c-axis direction while making use of elastic distortion due to the misfit. Likewise, it is possible to obtain a ferroelectric film of several tens of nanometers in thickness having the (001) direction.
In this regard, it is noted that the effect of elastic distortion due to the misfit becomes slender with increasing film thickness, because of being absorbed by rearrangement. When a thin film is used as a capacitor or the like, it is unnecessary to increase film thickness except for the purpose of reducing leakage. To use a ferroelectric thin film in the form of a piezoelectric film for thin-film bulk vibrators as an example, it is required to make use of resonance in the thickness direction of the thin film. To be more specific, a thickness of the order of at least several hundred nanometers is needed for obtaining a frequency range of 1 GHz to 5 GHz capable of taking full advantage of the thin-film bulk vibrator, although varying with the frequency used. At such a thickness, the effect of elastic distortion due to the misfit vanishes into almost nothing; any satisfactory piezoelectric properties are no longer achieved. If use is made of a process wherein, as shown in JP-A 10-287494 assigned to the applicant, a ferroelectric thin film and an electrically conductive oxide thin film are repeatedly laminated together at a thickness at which the elastic distortion in the ferroelectric thin film is not relaxed, then the thickness of the ferroelectric layer may be effectively increased. However, this makes the fabrication process complicated, and offers some problems as well. For instance, resonance characteristics become worse due to the presence of many layer interfaces in the ferroelectric thin film. It is thus required to improve the 90xc2x0 domain structure in the ferroelectric film with the number of lamination close to a single layer, thereby bringing ferroelectric crystals as close to the (001) uniaxial orientation as possible.
As already mentioned, the two-dimensional large tensile stresses remain within the plane of a current ferroelectric thin film formed on an Si single crystal substrate. Especially with a ferroelectric thin film having a thickness of as large as several hundred nanometers, it is not possible to obtain sufficient spontaneous polarization values or piezoelectric properties.
It is therefore an object of the invention to provide a multilayer thin film including a preferentially (001) oriented ferroelectric thin film having any desired thickness on an Si substrate, and its fabrication process. If a preferentially (001) oriented ferroelectric thin film having any desired thickness can be formed on an Si single crystal substrate that is a semiconductor substrate, it is then very advantageously applicable to a variety of electron devices inclusive of thin-film vibrators, thin-film VCOs and thin-film filters used for mobile communications equipment, thin-film piezoelectric devices used for liquid injectors, etc., semiconductor memories, thin-film ferroelectric devices such as infrared sensors or recording media for recording information by inversion of polarization of a ferroelectric material by an AFM (atomic force microscope) probe or the like.
Such an object is attained by the following embodiments (1) to (12) of the invention.
(1) A multilayer thin film formed on a substrate by epitaxial growth, which comprises a buffer layer comprising an oxide and a ferroelectric thin film, with a metal thin film and an oxide thin film formed in this order between said buffer layer and said ferroelectric thin film.
(2) The multilayer thin film of (1) above, wherein said oxide thin film has electrical conductivity.
(3) The multilayer thin film of (1) or (2) above, wherein an oxide thin film is formed on said ferroelectric thin film.
(4) The multi layer thin film of any one of (1) to (3) above, wherein said oxide thin film is a perovskite oxide.
(5) The multilayer thin film of any one of (1) to (4) above, wherein an a-axis lattice constant of a material used for said oxide thin film is smaller than an a-axis lattice constant of a material used for said ferroelectric thin film.
(6) The multilayer thin film of any one of (1) to (5) above, wherein said metal thin film contains at least one of Pt, Ir, Pd, Rh and Au.
(7) The multilayer thin film of any one of (1) to (6) above, wherein said metal thin film has a thickness of 50 to 500 nm.
(8) The multilayer thin film of any one of (1) to (7) above, wherein said ferroelectric thin film contains Pb and Ti.
(9) The multilayer thin film of any one of (1) to (8) above, wherein said buffer thin film contains zirconium oxide, a rare earth element oxide or zirconium oxide with a part of Zr substituted by a rare earth element or an alkaline earth element.
(10) The multilayer thin film of anyone of (1) to (9) above, wherein said substrate is an Si (100) single crystal substrate.
(11) An electron device comprising a multilayer thin film as recited in any one of (1) to (10) above.
(12) A process for fabricating a multilayer thin film by:
forming a buffer layer comprising an oxide on a substrate by epitaxial growth,
forming a platinum metal thin film thereon, and then forming an electrically conductive perovskite oxide thin film on the platinum metal thin film by epitaxial growth, and
forming a ferroelectric thin film on the perovskite oxide thin film by epitaxial growth.
By forming a metal thin film and an oxide thin film between the Si substrate and the ferroelectric thin film in this order, stresses applied on the ferroelectric thin film are relaxed, thereby obtaining a preferentially (001) oriented film.
In what follows, the action of the invention is explained.
A PZT formed on an Si substrate is likely to have a domain structure with the (001) orientation coexisting with the (100) orientation by virtue of tensile stresses due to the Si substrate during the cooling process from the film-formation temperature down to room temperature. Even after domain formation, the tensile stresses increase continuously in the cooling process, and so cause two-dimensional elastic deformation of the film. This in turn decreases the lattice constant of the film in a direction vertical with respect to a film plane, and so causes the properties of the (001) oriented portion to become further worse. To avoid such domain formation and deformation due to the tensile stresses, JP-A""s 10-223476 and 11-26296 published under the name of the applicant show a process wherein during film formation, a film is compressed using the misfit, thereby compensating for the tensile stresses during cooling. However, this process is found to be only effective at up to several tens of nanometers where elastic distortion due to the misfit does not vanish. One possible effective way for reducing tensile stresses at larger film thicknesses is to absorb the tensile stresses on the ferroelectric thin film by the underlying structure.
When a tetragonal ferroelectric material is formed directly on a metal thin film such as a Pt thin film, it is possible to relax stresses due to a thermal expansion difference between the Si substrate and the ferroelectric film, because the metal thin film is amenable to plastic deformation. Since such stresses are not completely relaxed, however, tensile stresses are still applied to the ferroelectric, and so domain formation and a decrease in the lattice constant in the vertical direction are unavoidable. When the lattice constant of the metal thin film within the film plane is smaller than that of the ferroelectric as found in a Pt-PZT combination, the ferroelectric is subject to elastic distortion due to the misfit between the metal thin film and the ferroelectric thin film. For this reason, it is expected that the lattice constant of the ferroelectric in the vertical direction with respect to the film plane may be extended or the proportion of the (001) domains with respect to the (100) domains may increase. However, no sufficient effect is still obtainable because a metal is generally easier to deform than a ferroelectric material; that is, most of elastic deformation is absorbed by deformation and rearrangement in the metal thin film.
On the other hand, when a ferroelectric thin film is formed on an oxide thin film such as an electrically conductive perovskite oxide formed directly, or via an oxide buffer layer, on an Si substrate, it is possible to prevent the ferroelectric thin film from having domains or deforming by the effect of distortion due to the misfit on condition that the ferroelectric thin film has a small film thickness. As the film thickness increases to several hundred of nanometers or more, however, this effect disappears; in other words, the film grows at the lattice constant inherent in the ferroelectric thin film material. As this multilayer thin film is cooled down to room temperature, tensile stresses resulting from the substrate are transmitted to the ferroelectric thin film without being substantially relaxed because of the absence of any metal or other soft material layer between the ferroelectric thin film and the substrate. As a result, the domains formed or deformation rather becomes large. In addition, another problem such as cracking may possibly arise.
According to the present invention, therefore, between the ferroelectric thin film and the substrate a layer comprising a metal thin film and an oxide layer having a coefficient of thermal expansion close to or higher than that of the aforesaid ferroelectric thin film are interposed. The oxide layer is much more contracted than the Si substrate during cooling because the coefficient of thermal expansion of the oxide layer is larger than that of Si. If this layer is a cubic crystal system, the degree of contraction can be much more increased, because the layer cannot have any domain; stresses cannot be relaxed by domains formed within the film. In addition, the oxide layer is less susceptible to deformation, and so contraction-inducing force is efficiently transmitted to the underlying metal layer without being absorbed by deformation within the film. Consequently, the stresses between the Si substrate and the oxide layer are absorbed by rearrangement within the metal layer interposed therebetween or slips in the vicinity of interfaces. As a result, the tensile stresses coming from the substrate and acting on the ferroelectric film become weaker as compared with the case where the underlying layer is composed solely of the metal thin film, and so it is possible to prevent the ferroelectric film from having domains or reduce a decrease in the lattice constant of the ferroelectric film in the vertical direction. Further, this effect is much more enhanced, if another oxide thin film is formed on the ferroelectric film, because the ferroelectric film can receive compressive stresses from the overlying film.
Furthermore, when the a-axis lattice constant of the material used for the oxide thin film is smaller than the a-axis lattice constant of the material used for the overlying ferroelectric thin film, the effect of elastic distortion due to the misfit can be added to the ferroelectric thin film, so that the ferroelectric film can be elongated in the c-axis direction , with efficient (001) orientation.